Again, the payoff for Republicans is in the makeup of the state's delegations: In those six states, Republicans picked up about 11 more seats than you'd expect from simply looking at the parties' vote shares.

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Both sides do this, it's just that Republicans are a little bit better at it from what I can work out in the article.

Once drawn and the gerrymandered lot get in it tends to entrench the situation. The effect was put as high as 7 or 8% in some states (?) somewhere that was mentioned, which would be about right - the other side are handicapped before they start and will need huge swings in the other seats to have any chance there.

NZ's Representation Commission has done an excellent job over the years compared to the districting system in the US.